When generating an app with the --dev flag, mount the local rails rep…

…o in the devcontainer

This is useful for testing changes to how the devcontainer is generated, and will be necessary for creating a CI workflow for testing the devcontainer setup.

railties/lib/rails/generators/devcontainer.rb

@@ -39,6 +39,16 @@ def devcontainer_volumes
           @devcontainer_volumes
         end
 
+        def devcontainer_mounts
+          return @devcontainer_mounts if @devcontainer_mounts
+
+          @devcontainer_mounts = []
+
+          @devcontainer_mounts << local_rails_mount if options.dev?
+
+          @devcontainer_mounts
+        end
+
         def devcontainer_needs_redis?
           !(options.skip_action_cable? && options.skip_active_job?)
         end
@@ -127,6 +137,14 @@ def mariadb_service
         def db_service_names
           ["mysql", "mariadb", "postgres"]
         end
+
+      def local_rails_mount
+        {
+          type: "bind",
+          source: Rails::Generators::RAILS_DEV_PATH,
+          target: Rails::Generators::RAILS_DEV_PATH
+        }
+      end
     end
   end
 end

railties/lib/rails/generators/rails/app/templates/.devcontainer/devcontainer.json.tt

@@ -29,6 +29,12 @@
   // Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
   // "remoteUser": "root",
 
+<%- if !devcontainer_mounts.empty? -%>
+  "mounts": [
+    <%= devcontainer_mounts.map { |mount| "{\n      " + mount.map { |key, value| "\"#{key}\": \"#{value}\"" }.join(",\n      ") + "\n    }" }.join(",\n    ") %>
+  ],
+<%- end -%>
+
   // Use 'postCreateCommand' to run commands after the container is created.
   "postCreateCommand": "bin/setup"
 }

railties/test/generators/app_generator_test.rb

@@ -1415,6 +1415,18 @@ def test_devcontainer_no_depends_on_when_no_dependencies
     end
   end
 
+  def test_devcontainer_dev_flag_mounts_local_rails_repo
+    run_generator_using_prerelease [ destination_root, "--dev" ]
+
+    assert_devcontainer_json_file do |devcontainer_config|
+      rails_mount = devcontainer_config["mounts"].sole
+
+      assert_equal "bind", rails_mount["type"]
+      assert_equal Rails::Generators::RAILS_DEV_PATH, rails_mount["source"]
+      assert_equal Rails::Generators::RAILS_DEV_PATH, rails_mount["target"]
+    end
+  end
+
   def test_skip_devcontainer
     run_generator [ destination_root, "--skip-devcontainer" ]
 

railties/test/generators/generators_test_helper.rb

@@ -122,6 +122,12 @@ def assert_compose_file
     end
   end
 
+  def assert_devcontainer_json_file
+    assert_file ".devcontainer/devcontainer.json" do |content|
+      yield JSON.load(content)
+    end
+  end
+
   private
     def gemfile_locals
       {